Design of an existing vehicle alternating current generator is carried out in such a way that each part used is kept at or below an allowable temperature by output power of the generator being adjusted (adjusted in a downward direction), thereby reducing heat generated by the generator itself, as a method of realizing durability in a high temperature environment. In order to realize this, however, it is necessary to adjust mechanical design specifications, those being magnetomotive force of a generator field coil (coil wire diameter, number of turns), an air gap between a rotor and stator, and transmission efficiency such as a cooling fan of the generator, because of which man hours and costs for design change are high.
Also, there is a correlation between the field current flow and output power of a generator, because of which, when the mechanical design specifications of the field coil are changed in order to realize generator durability in a higher temperature environment, there is a problem in that output decreases over the whole rotation speed range and temperature range of the generator, and output power decreases invariably over the whole range of use, as shown in FIG. 6 and FIG. 7.
Meanwhile, there is a proposal whereby an electricity supply continuity rate restriction value is switched in order that the continuity rate of a supply of electricity to the field coil is restricted when the vehicle is cruising in a steady state, and the electricity supply continuity rate is increased when the vehicle decelerates, such as when regenerative power is generated, whereby a large amount of regenerative energy is obtained without the generator size increasing (refer to PTL 1).
However, PTL 1 is such that restriction of the electricity supply continuity rate is released only in a rotation range in which regenerative power generation is necessary (when decelerating), and no consideration is given to high temperature durability in other operating ranges. Also, the temperature of the generator rises abnormally in an environment wherein regenerative power generation is carried out frequently, with the result that characteristics of the generator are caused to depreciate.